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joa
10-24-2004, 04:26 PM
David, a while back there was a thread about painting on linen, and you said this:

When acrylic paint cures, water exits both through the surface of the paint and through the back -- and as the paint dries there will be a period of time (depending on temperature, humidity, etc.) during which the water will move in both directions. And that is when impurities can be moved from the support to the paint. Many acrylic mediums will prevent that from happening (gesso alone won't) -- I use GAC 100 because it has been specifically tested for blocking discoloration.

I got some GAC 100 because in the humidity here I worry about stains moving from the support to the paint. I added some color to it so that I won't miss any spots on the canvas. Here's my question:

The directions say to "mix desired quantity of this medium with" other mediums. "Thin with water."

The GAC 100 is not too expensive, but I have to order it online and pay for shipping, so I would like to thin it. How much can I thin it and still maintain protection against SID?

Thanks,
Jo

JamieWG
10-24-2004, 05:38 PM
The GAC 100 is not too expensive, but I have to order it online and pay for shipping, so I would like to thin it. How much can I thin it and still maintain protection against SID?

Thanks,
Jo

Jo, the Golden company is the one that has done the majority of the research on SID. They have great information on their website about it. They do not say anything about thinning the GAC100 when using it to control SID. I use it on masonite/hardboard, and always do two coats, including the exposed sides of the panels, at full strength. It's pretty thin to begin with, IMO. Here is the link to that page of their website:


http://www.goldenpaints.com/technicaldata/prepsupp.php

And a quote from it:

Controlling SID in Acrylic Paints
'G' with GAC 100 application Support Induced Discoloration (SID) is a phenomenon that occurs in acrylic paints and mediums. Many common artist supports have impurities that can discolor a translucent acrylic gel layer or color glaze, and a size must be applied before gessoing to ensure the products stay clear as the films dry.

As a paint film cures, the water exits two ways: through the surface of the paint and through the back of the support, if porous enough. Canvas, linen, wood and masonite are all porous enough to allow water to absorb into them. During this drying process, the water is actually in equilibrium moving back and forth between the acrylic paint and the support. The water extracts water-soluble impurities such as dirt, sap, starches, etc., from the support and deposits them into the acrylic film. The result is a discolored (typically amber) film, with the degree of discoloration dependent on the amount of contaminants deposited and the inherent level of inpurities in the support.

SID contamination often goes undetected. In most cases, the paints applied contain a sufficient level of pigment, thus a strong enough color, to conceal the yellowing. However, in a transparent glaze and especially in thick translucent gel layers, SID becomes quite noticeable. SID can transform the appearance of an Ultramarine Blue glaze into a lower chroma, greenish color. Gesso alone will not stop SID, and different gels and mediums have varying degrees of blocking capabilities. The best product Golden Artist Colors produces to prevent SID is GAC 100. This thin medium works best when 2 or more coats are applied directly into the support. Once dry, the canvas can then be primed and subsequently painted with less potential for discoloration. Pre-primed canvases can be sealed with GAC 100 as well. Apply one or two coats onto the surface, and follow with at least one coat of gesso to regain tooth if needed.

I hope this helps.....

[Jo, I know you directed the question here to David.....but just figured I'd give you what I know as long as I saw it....]

Jamie

joa
10-24-2004, 06:05 PM
Thanks, Jamie--

I really didn't care who answered it, but that thread was the only one I had seen where this product and its use was mentioned. I was hoping that if David didn't see my post that someone would.

I suppose you could put it over gesso, since they are saying that you can use it on primed canvas? I have a couple of canvas boards that I have already gessoed, and would like to use the GAC over it if that will still do the job. What do you think?

Again, thank you so much for your answers. I am going to visit Golden and read everything I can find!

Jo